Production of dissolving pulp from a lignocellulosic material requires selective removal of hemicellulose and lignin in the lignocellulosic material in order to increase the cellulose purity. As an index representing the cellulose purity, an α-cellulose content is generally used. As the content is larger, the quality of the dissolving pulp is higher. As methods of producing dissolving pulp, two methods, namely, an acidic sulfite cooking method and a hydrolysis-kraft cooking method have been known. In the acidic sulfite cooking method, large amounts of hemicellulose and lignin in a lignocellulosic material are removed at one time at a cooking process. In the hydrolysis-kraft cooking method, most of hemicellulose is removed by acid hydrolysis at a hydrolysis process, and a small amount of hemicellulose and most of lignin are then removed at a kraft cooking process. At the hydrolysis process, water is added to the lignocellulosic material, followed by heating, to eliminate an acetyl group in the hemicellulose, thereby generating acetic acid. Thus, the state automatically becomes acidic, and the acid hydrolysis advances. Therefore, the hydrolysis process is generally performed without external addition of acid (Patent Literature 1).